Update multiple_kernels.rst

Adding the two required namespaces in the code sample

Author: npotier

configuration/multiple_kernels.rst

@@ -117,6 +117,9 @@ resources::
     // src/Kernel.php
     namespace Shared;
 
+    // ...
+    use Symfony\Component\DependencyInjection\Loader\Configurator\ContainerConfigurator;
+    use Symfony\Component\Routing\Loader\Configurator\RoutingConfigurator;
     // ...
 
     class Kernel extends BaseKernel